1. Field of the Invention
This invention relates to the extraction of glass ribbon from a flotation furnace.
2. Description Of The Prior Art
The manufacture of glass ribbon by casting glass on a molten metal bath contained in a flotation furnace is well known. In the known process, molten glass is poured onto a bath of molten metal to form a flat ribbon of glass. The metal bath is shielded by a protective reducing atmosphere composed of hydrogen and nitrogen. The protective atmosphere is retained within a leak-proof enclosure at slightly greater than atmospheric pressure. The glass ribbon is drawn from the flotation furnace by special extraction rollers whose height may be accurately positioned with respect to the level of the metal bath. These rollers also convey the glass ribbon to a flattening fixture for further treatment.
To ensure that the metal bath is protected by the reducing atmosphere, losses of the protective atmosphere as well as inflows of ambient air to the flotation furnace must be minimized. These losses and unwanted inflows are acute at either end of the metal bath. To maximize retention of the protective reducing atmosphere, the prior art devices have their extraction rollers located in a lock-chamber of appreciable length which contains the same protective atmosphere as above the metal bath. The upper portion of the lock-chamber is barred by a succession of leakproof curtains arranged transversely above the extraction rollers. The lower portion of the lock-chamber is barred by leak-proof joints positioned underneath the lower generatrix of the rollers. The curtains and joints form a leak-proof barrier which prevents the ambient atmosphere from contaminating the protective atmosphere. Thus, the leak-proof barrier protects the metal bath from oxidation. Lock-chambers of this type have several drawbacks. Access to various members such as the extraction rollers is difficult, making maintenance and replacement of the rollers arduous. Furthermore, the appreciable length of these lock-chambers results in excessive heat loss from the glass ribbon as it passes through the lock-chamber. Consequently, the temperature of the glass ribbon exiting the lock-chamber is often too low to permit further processing of the glass such as treating it with metal oxide deposits.
Other prior art devices have the extraction rollers positioned downstream of the lock-chamber and exposed to the ambivient air. Access to the rollers is facilitated by this arrangement but a substantial temperature gradient between the furnace exit and the flattening fixture entrance occurs. This gradient may alter the quality of the manufactured glass ribbon.
We have invented an apparatus for extracting glass ribbon from the egress of a flotation furnace which provides easy access to the extraction rollers, retains the glass ribbon exiting the furnace at a temperature sufficiently high to permit further processing and maintains a uniform thermal transition of the glass ribbon passing from the furnace exit to the flattening fixture entrance.